It has been recognized that particles of insecticide may be used effectively to control insects while simultaneously controlling the speed of release of the insecticide and thus minimizing any undesirable effects. Traditionally, processes for preparing such particles have involved co-melting the pesticide with a barrier which may be a polymer. After the polymer and insecticide have been melted together, a grinding step takes place in which the particles are reduced to the desired size. This sort of technique is taught in Canadian Pat. No. 846,785 to G. C. Allen, in Coppedge et al., J. Economic Entomology 68, 508 (1976) and in Canadian Patent No. 787,777.
The co-melting technique is subject to disadvantages. For instance, one is limited with regard to the polymers which may be utilized because high melting or non-melting polymers are inoperable. Thus, a large number of polymers which may prove useful in a controlled release particle must, unfortunately, be eliminated from consideration. Another disadvantage stems from the fact that melting may deleteriously affect certain insecticides. Finally, this process necessitates a grinding step, an oftentimes difficult step which can adversely affect the structural integrity of the particles.
Other techniques which have been used to prepare insecticide particles include microencapsulation which tends to be quite expensive and which usually results in the presence of by-products in the insecticidal formulation itself. Other known methods require tedious control during production and have not always produced desirable particulate products. These methods are difficult to apply to methomyl because of the difficulty in matching the solubility characteristics of methomyl to the physical conditions required in these processes.
Thus, a need exists for a process for producing particles of insecticide, such as methomyl, embedded in a polymer. These particles should be efficiently and rapidly produced and should release the insecticide in a controlled fashion so that desired crops such as cotton are not injured. In addition, it is desired that the particles exhibit a combination of high initial and long residual insecticidal activity. This combination of characteristics are not readily found in available controlled release pesticide products.